1. Field of the Invention
The present invention relates to an X-ray mask using an X-ray absorbing film such as tantalum (Ta), tungsten (W), or tantalum-tungsten alloy, to a method of manufacturing the same and to an exposure method using the X-ray mask.
2. Description of the Related Art
As a very large scale integrated circuit (VLSI) is more densified, a super-fine fabrication technique is being demanded. To form a pattern having a line width of 0.5 .mu.m by lithography using an X-ray mask, an error due to a strain of an X-ray absorbing pattern of the X-ray mask should be decreased at least to 0.05 .mu.m or less.
The X-ray mask is conventionally formed according to the process shown by the sectional views in FIG. 1. The following is the brief description of the process.
First, as shown in FIG. 1(a), a membrane 2 composed of silicon carbide (SIC) is formed on the top surface of a silicon (Si) substrate 1 by the low pressure CVD method. After a SiC film 3 is formed on the underside of the substrate, an opening 4 to expose the central portion of the silicon substrate is formed by patterning the SiC film 3 by lithography.
Then, as shown in FIG. 1(c), an X-ray absorbing film 5 composed of a material such as W or Ta is grown on the membrane 2 by the sputtering method, and then, after the Si substrate 1 is bonded with a support frame 6 through the SiC film 3, a window 7 for exposing the membrane 2 is formed by etching the Si substrate 1 through the opening 4 of the SiC film 3.
Then, as shown in FIG. 1(d), after a photoresist pattern 8 is formed on the X-ray absorbing film 5, patterning is applied to the X-ray absorbing film 5 by lithography techniques with the pattern 8 as a mask. And, when the photoresist pattern 8 is removed, the X-ray mask as shown in FIG. 1(e) is completed. Major strains in the X-ray mask manufacturing process described above are generated when patterning the X-ray absorbing film 5. The main cause of the strains is the stress of the X-ray absorbing film 5. To minimize the stress generated in depositing an X-ray absorber, various parameters such as the film-forming chamber pressure, discharge power and substrate temperature have been conventionally adjusted to form a film.
In general, the most effective method to control the stress of the X-ray absorbing film 5 is to change the pressure in the film-forming chamber in forming a film. However, as the result of controlling the pressure in the film-forming chamber in order to bring the stress of the X-ray absorbing film 5 close to zero, the film stress is greatly changed due to a slight change of the pressure. Therefore, the controllability is low.
For example, when it is assumed that the material of the X-ray absorbing film is Ta, as the result of forming an X-ray absorbing film of Ta having a film stress close to zero by adjusting the pressure in the film-forming chamber, the stress is greatly changed to a compressive stress due to thermal treatment and change with the passage of time as shown in FIG. 2. Therefore, the X-ray absorbing pattern formed by the Ta film has a problem that it is deformed due to the compressive stress. The compressive stress is expressed as a negative value in FIG. 2.
For example, the official gazette of Japanese Patent Laid-Open No. 34312/1991 proposes a method of implanting ions of elements such as argon (Ar) and krypton (Kr) into an X-ray absorbing film in order to decrease the change of compressive stress generated after forming a film. Though this method makes it possible to decrease the change of the stress of the X-ray absorbing film to a certain extent, further suppression of the stress change is desired. Moreover, this official gazette describes that ions are implanted in the X-ray absorbing film without thermally treating the film after the film is formed.